|Some hp are lost...|
The basic of it is that the maximum number of Hit Points doesn't need to be fixed once and for all at character creation and at level progression, indeed we can think of the maximum number of hp as a number that can change for various reason (training, disease, poisons, undead, curses and more).
Let see the details coming from this change of perspective.
Gaining a level (and HP)
When a character gains a new level he can just roll his hp as usual or chose to train to get the new level.
Self-Training (takes 3 days/level to be gained and cost 500 GP x level to be gained): this assures that 1 can be rolled again
|A true master|
Training for more HP
A character can train whenever he wants to improve his hit points, but it will become harder and harder to gain more hp without gaining a new level. The time required is 2 week x level and the cost is 5000 GP x level. The effect of such training is to roll again all HD and keep the higher between current HP and rolled HP (eg a 5 level fighter, constitution 10, with 5d8 HD and 25 hp could train for 10 weeks and roll 5d8, if he rolls 25 or less he will stay at 25 hp, if rolls 26 or more will keep the new total hp).
At first level you can assume that the character was trained and trained and trained until his master, or fate, sent him away. So you (the DM) can decide to give 1 or more roll and at least half maximum hp or just give maximum hp to starting character (without breaking the rules).
Notice that such a training can be very different from class to class. For example a fighter could train in parries and to roll with blows, a magic-user could train to cast stronger protective enchantments and charms, a cleric could make new oaths and more martial training, a thief could train in new acrobatics to escape from harm.
And now the bad news
Since it's now possible to raise the maximum number of HP it should also be possible to see this number lowered.
Disease: whenever a character is hit by some disease the maximum number can be lowered even to 1hp for each HD, maybe an additional saving throw should be used to simulate such a possibility.
Laziness: a character who stay at home without going around adventuring should train or lose his edge (each month without training could lower the maximum hp by 1 for each HD, to keep fit you must train for at least 4 days each month, or 1 hour each day - the cost of such training is 100 gp/level each month).
Curses: some curses could lower the maximum number of HP for each HD.
Undead: the energy draining of undead could also lower the maximum HP for each HD.
Elite and paragon monsters
Such rules can be also used to represent elite and paragon monster, some chosen unit of monsters could have at least 4hp for each HD, and paragon monster could have at least 6hp for each HD. While the normal rules can be used to represent your run of the mill monster or the possibilities that the encountered monster is already wounded or tired.
Any feedback is welcome!